Case hardening composition



Patented Jan. 23, 1940 UNITED STATES The present invention comprises a salt bath for .case hardening iron, steel, and alloys thereof.

The principal object of the present invention is to provide an efllcient, economical case hardening salt bath which is operative at relatively low temperatures and which leaves a water soluble residue on the work.

Another object of the present invention is a salt bath wherein silicon carbide is one of the sources for the carbon element for the case hardening operations.

The preferred formulae for the case hardening salt bath comprising the present invention is a composition of sodium, potassium, or calcium chloride, 22% to 45%; sodium, potassium, or calcium cyanide, from a small amount up to 41%; sodium, potassium, strontium, or barium carbonate, 30% to 45%; sodium, potassium, barium, or calcium fluoride, 2% to 5%; and silicon carbide, 1% to 10% and adapted to be run at temperature ranges from 1450 to 1650" F. Since it is desirable that the surface of this salt bath when liquid shall be protected from the atmospheric oxygen, ground or flaked graphite may be mixed with the composition in order that it will float to the top of the bath and form a protecting layer thereon when the materials comprising the composition are melted in a suitable pot. The chloride constituent of the bath tends to produce fluidity and also acts as a carrier for other materials and improves the washing qualtities of the composition. The cyanide constituent produces a portion of the carbon element that is absorbed by the metal being treated. The cyanide also produces nitrogen which is useful in some cases. The fluoride acts as a catalyst to increase the chemical activity of the cyanide radical. The carbonate constituent of the bath tends to lower the melting point of the bath and also helps to maintain the bath on the alkaline side so that it reduces the tendency of the treated metal to rust. Furthermore, the carbonate reacts with the silicon carbide particles. The silicon carbide remains in the bath in solid, crystalline form when the other bath constituents have all been melted. The reaction of the carbonate constituent with the silicon carbide frees the carbon element of the carbide, and also forms a water soluble silicate. The entire group of salts forming the composition are salts which are easily soluble in-water, after reaction has occurred, and, therefore, the residue which is dragged out of the bath by the work is easily removed from the work by washing and as above stated, the residue is on the alkaline side so that the tendency to rust the work is avoided.

The above formula may also be modified by increasing or decreasing the cyanide content while decreasing or increasing the carbonate content for the reason that as the cyanide breaks down in use, carbonates are formed and these carbonates PATENT OFF ICE Q CASE HARPENING COMPOSITION No Drawing. Application October 26, 1938 Serial No. 237,005

4 Claims. 7 (Cl. 148-30) in turn react with the silicon carbide to release the carbon element from the carbide and to form water soluble silicates.

Furthermore, since cyanides contain nitrogen and there are conditions in which case hardening is desired without introducing nitrogen into the metal, this desirable condition may be obtained by utilization of a bath, accordng to the present invention, in which the cyanide is reduced to a minimum, in which case the carbon element for the carburizing action comes principally from the silicon carbide due to the reaction of the carbonate upon the silicon carbide. The characteristic of the case thus produced is one wherein-marbon is introduced into the metal and substantially no nitrogen is introduced. Where sodium cyanide and silicon carbide are compared in equal weights, a larger percentage of the carbon element will be available 'from the silicon carbide than from the sodium cyanide.

The following are preferred percentages of specific compositions? Series #1 Per cent Potassium chloride--. 27.3 Sodium cyanide -1 24.3 Sodium car 43.7 Sodium fluoride 3.0 Silicon carbide 1.0 Pure plumbago or graphite .7

Series #2 Per cent Potassium chloride 24.0 Sodium cyanide 29.33 Sodium carbonate 42.0 Sodium fluoride 3.0 Silicon carbide 1.0 Pure plumbago or graphite .67

Series #3 Per cent Potassium h rid 24.0 Sodium cy d 35.08 Sodium carb n 36.0 Sodium fluoride 3.0 Silicon carbide 1.25 Pure plumbago or graphite .67

Series #4 H Per cent Potassium chloride 24.0 Sodium cy n 39.33 Sodium carbonate 32.0 Sodium fluoride 3.0 Silicon carbide 1.0 Pure plumbago or graphite .67

The formula above disclosed and designated Series #1 is designed for a maximum depth of case of .007 at temperatures from 1475 to 1600 F. for a half hour run, and is intended primarily for surface hardening.

The formula'above designated as Series #2 is designed for a slightly deeper case and will produce a case of .010 at temperatures of 1475 to 1600 F. when run for a half hour.

The formula. designated as Series #3 is designed to produce a depth of case of .015 at temperatures from 1475 to 1650 F. when run for one hour.

The formula Series #4 is intended to produce a depth of case that is considerably deeper thanthose produced by the foregoing formulae. This formula, Series #4, may also be run from 1475 to 1650 F., and when run at 1650 F., this composition will produce a .018 to .020 deep case in one hour; a .025 to .028 deep case in two hours; a .032 to .035 deep case in three hours; and a .040 to .045 deep case in four hours.

The reaction of these formulae containing silicon carbide is somewhat dependent upon the size of grain of the silicon carbide, the finer the grain, the more reactive is the composition, and the preferred size of grain is that which will pass through a one hundred to the inch mesh screen. However, satisfactory results are obtained with silicon carbide particles varying in size range from those passing a twenty-five mesh to the inch to a two hundred mesh to the inch screens. The fluoride in the composition also controls the speed of reaction of the composition and apparently acts as a catalyst. The composition reacts more rapidly with fluoride therein than where the fluoride is absent and the amount of fluoride necessary to form a correct balance of the formula for proper reaction is substantially in proportion to the amount above specified in this bath.

The rate of carburization is more uniform with the baths according to the present invention than one wherein cyanide alone is relied upon. Furthermore, the present bath does not gasify and there is little or no loss of carbon monoxide or carbon dioxide gas from the bath. These gases appear to be trapped in the bath and are utilized in a most efllcient manner in carburizing the work.

What I claim is:

1. A case hardening composition comprising an alkali metal or an alkaline earth metal chloride 22% to 45%, an alkali metal or an alkaline earth metal carbonate 30% to 45%, and silicon carbide 1% to 10%, and sodium cyanide 25% to 41%, with the size of the silicon carbide particles being between one-twenty-flfth to one-twohundredth of an inch square.

2. A case hardening composition operative at temperatures between 1450" and 1650 F., and

comprising an alkali metal or an alkaline earth metal chl0ride,22% to 45%; an alkali metal or an alkaline earth metal cyanide 24% to 41%,

an alkali metal or an alkaline earth metal carbonate to an alkali metal or an alkaline earth metal fluoride 2% to 5%, and silicon carbide 1% to 10%.

3. A case hardening composition comprising potassium chloride 22% to 45%; sodium cyanide 24% to 41%; sodium carbonate 30% to 45%; sodium fluoride 2% to 5%; and silicon carbide 1% to 10%.

4. A case hardening composition comprising an alkali metal chloride 22% to 45%, an alkali metal carbonate 30% to 45%, an alkali metal fluoride 2% to 5%, silicon carbide 1% to 10% of the total mass, and sodium cyanide in an effective amount up to 41% of the total mass.

HAIG SOLAKIAN. 

